1. Field of the Invention
The present invention relates to an up-converter or a down-converter in a communication system, and more particularly, to a mixer capable of detecting or controlling a common mode voltage thereof.
2. Description of the Prior Art
An up-converter or a down-converter in a communication system generally includes a mixer, and a passive mixer is typically utilized. A switching transistor of a passive mixer operates in a non-saturation region. The passive mixer is utilized in a direct down conversion receiver because it is cost-effective.
The utilization of operational amplifiers in mixers of the prior art results in more noises. Some complementary mixers may balance input signals, however, voltage gains of mixers are influenced by manufacture processes and temperature variations. One method to solve this problem is increasing the linearity of a mixer by increasing a linear region of transconductance of the mixer. Unfortunately, the influence to the voltage gain of the mixers by the manufacturing processes and temperature variations remains. Another method is increasing transconductance of the mixer with a feedback circuit to increase the linearity of the mixer. Unfortunately, the same problem of the influence to the voltage gain of the mixer caused by the manufacturing processes and temperature variations still exists.